Tanning



' complexes. I Sodium hexametaphosphate probably has a Patented Dec. 13, 1938 UNITED STATES PATENT OFFICE 7 TANNING Ralph E. Hall, Mount Lebanon, 1a., assignor to Hall Laboratories, Inc., poration oi. Pennsylvania Pittsburgh, Pa, a cor-,

No Drawing. Application June 1 8, 1936, Serial No. 85,963. Renewed February 2, 1938 8 Claims.

part of my application Serial No. 685,754, fi1ed August 18,1933.

Iron combines with tanning bodies to form dark colored compounds which, if present in the tanning liquors or materials to be tanned; cause discoloration in the tanned materials. The waste liquors from paper making processes con- .tain tannin bodies, sometimes called "pseudo tannins. sulphite liquors, are employed for tanning purposes, The waste liquors are usually treated and concentrated to form tan 'ng compounds in the. form of solid bodies or xtracts. In the production of such waste liquors in the paper mills, they are frequently contaminated with small amounts of iron impurities which combine with the tanning bodies in the liquors to form dark colored compounds which discolor the leather being tanned."

I have found that such discoloration may be prevented by employing analkali-metal hexametaphosphate, for example sodium .hexametaphosphate in such iron contaminated tanning liquors. The hexametaphosphates singular property of converting the iron into soluble, very slightly ionized form, thereby preventing the iron from combining with the tanning bodies to produce the dark discolorations. The effect of the hexametaphosphates is to sequester the iron in soluble, slightly ionized molecular. constitution indicated bv the formula Na2(Na4P0O1a). Ferrous iron combines with the hexametaphosphate to produce a but slightly ionized complex molecule believed to have the molecular constitution indicated by Na2(Fe2PeO1a-).

in which-the radical F84P18054 is very slightly ionized.

As indicatedby these formulae, the effect of the hexametaphosphate is to sequester the iron in the form of but very slightly ionized complexes, so that the'solution has exceedingly low These liquors, particularly the waste have the ionic concentration of iron. This ionic concen- I tration is smaller than the concentration of ionic iron in equilibrium with the tanning bodies, and accordingly, the iron will remain in solution and will not combine with the tanning bodies to form the dark discoloring compounds.- Since the amount of iron in the tanning liquors is-usually relatively small, only a small amount of the hexametaphosphate need be ordinarily used. It

is preferred to add the sodium hexametaphosphate in some excess of that theoretically required, to combine with iron according to the above formulas. I have found that an amount of sodium hexametaphosphate of about 16 to 17 times that of the weight of the iron present will hold the iron against discoloring combination with the tanning bodies in tanning liquors having a pH value of not overabout 6 to '7. An increase in pH value results in a sharpdncrease in the amount of metaphosphate required until at a pH yalue of ver about 8 the metaphosphate no longer so hol the iron.

Fortunately, the pH values, of the tanning liquors lie within the limits within which the hexametaphosphates can effectively repress the iron. Usually, an excess somewhat above these amounts is employed as a safeguard, which is not objectionable since only a small amount of the hexametaphosphate usually need be added because the amountof iron impurities is usually relatively low.

The hexametaphosphate may be added to the waste liquors from the paper making operations during the preparation of these liquors for the tanning industry, and a compound, either as a solid or as an extract, furnished to the tanner containing sufflcient hexametaph'osphate to counteract such iron impurities as may be present,

in such tanning compound. Or the tanner may add the hexanzetaphosphate to the tanning liquor in the tanning vats. Y I

The hexametaphosphate may be employed with the natural tanning liquors, such as those produced from bark or wood, and will serve to prevent'discoloration due to iron compounds which maybe present in the water, the tanning compounds or the leathers to be tanned.

Since iron forms dark colored compounds with the pseudo tannin bodies such as are presentin the waste liquors from paper making, as well as the. tannins proper, when I speak of tanning liquors or tanningbodies; I intend to include the pseudo tannins as well as' the tannins proper.

The use of alkali-metal hexametaphosphates in tanning serves other important functions in addition to preventing discoloration by iron compounds. One of the principal difliculties associated with tanning is the appearance of calcium soaps on the surface of the leather. These calcium soaps come from withinthe hide, being formed during the liming process. The hexametaphosphates have the property of dissolving these calcium soaps, thereby allowing the tanning liquor to act more eflectively on the leather. Even though hides which have been limed are washed before the tanning operation, the removal of calcium is frequently incomplete. In such cases the lime soaps which have been formed by the reaction between the lime and the fats naturally present in the raw hides are not entirely removed before tanning. This frequently results in defects in the tanned hide called spewing" which consists in the appearance of grease spots on the surface of the leather, causing trouble in the further finishing operations. This spewing may be prevented by the use of alkalimetal hexametaphosphates in the tanning liquor due to the fact that the hexametaphosphates have the property of dissolving the lime soaps. Lime soaps, if present, not only cause defects in the finished leather, but prevent the effective penetration of the tanning'compound into the pores of the hide.

The speed and efliciency of penetration of the tanning liquor into the hides is increased by employing an alkali-metal hexametaphosphate in the tanning solution. The tanning of leather is commonly carried out at pH values below 5.0. At such low pH values, alkali-metal hexametaphosphates have the property of reacting with albumen to precipitate it and of reacting with other proteins commonly present in hides. It is believed that the effect of the hexametaphosphate in the tanning of leather rests in part at least upon the specific reactivity between the protein constituents of the hide and the hexametaphosphate. It is generally believed that during the process of tanning, addition compounds are formed by the tannins present in the tanning liquor and the protein constituents of the hides. Alkali-metal hexametaphosphates form insoluble addition compounds with soluble proteins, such as albumen, and insoluble proteins as well, and

it is believed therefore that the presence of socipitating action on albumens in acetic acid soluv tion. When to an acetic acid solution of egg white there is added a dilute solution of sodium hexametaphosphate, the albumen is quickly coagulated. This effect is not produced by sodium orthophosphate or sodium pyrophosphate. It is believed that in tanning operations where sodium hexametaphosphate is employed in tanning solutions having pH values below 5.0, it reacts with the albumen and proteins present in the hideto form addition compounds, thereby increasing the speed of penetration and effectiveness of the tannin bodies.

Normal sodium hexametaphosphate has a pH value in solution of about 6 and may be advantageously used as such in the tanning liquors. In general, the normal salts, the acid salts or even the acid may be used depending upon the pH value desired and the control of the pH value in the tanning liquors. The term alkali-metal hexametaphosphate when used without other qualification is intended to include the acid salts as well as the normal salts.

While I prefer to use sodium hexametaphosphate because of its greater cheapness and availability, the hexametaphosphates of the other alkali-metals may be employed, such for example as the salts of .potassium, ammonium, lithium,

etc.

I have described the preferred procedure in carrying out the invention and have set forth what I believe to be the correct theoretical explanation of the reactions involved. It is to be understood, however, that the invention is not limited to such preferred'procedure or by such theoretical explanations, but that the invention may be otherwise embodied or practiced within -the scope of the following claims.

I claim:

l. A process of tanning, which comprises subjecting the materials to be tanned to a tanning liquor containing an alkali-metal hexametaphosphate.

2. A process of tanning, which comprises subjecting the materials to be tanned to a tanning liquor containing sodium hexametaphosphate.

3. A tanning liquor containing an alkali-metal hexametaphosphate.

4. A tanning liquor containing sodium hexametaphosphate.

5. A tanning preparation containing an alkalimetal hexametaphosphate.

6. A tanning preparation comprising tannin bearing extracts and sodium metaphosphate.

7. A process ior tanning hides comprising immersing hides in a bath of tannin extract and a soluble dehydrated alkali metal metaphosphate.

8. A process for tanning hidescomprising immersing hides in a bath of tannin extract and a 

